* Add proc->p_token and use it to interlock signal-related operations.

* Remove the use of proc_token in various signal paths. Note that proc_token
is still used in conjuction with pfind().

* Remove the use of proc_token in CURSIG*()/issignal() sequences, which
also removes its use in the tsleep path and the syscall path. p->p_token
is use instead.

* Move the automatic interlock in the tsleep code to before the CURSIG code,
fixing a rare race where a SIGCHLD could race against a parent process
in sigsuspend(). Also acquire p->p_token here to interlock LWP_SINTR
handling.